Plasma generation is useful in a variety of semiconductor fabrication processes, for example plasma enhanced etching and deposition. Plasmas are generally produced from a low pressure gas by electric field ionization and generation of free electrons which ionize individual gas molecules through the transfer of kinetic energy via individual electron-gas molecule collisions. The electrons are commonly accelerated in an electric field, typically a radio frequency electric field.
Numerous techniques have been proposed to accelerate the electrons in an RF electric field. For example, U.S. Pat. No. 4,948,458 discloses a plasma generating device in which electrons are excited in a radio frequency field within a chamber using a planar antenna coil that is situated parallel to the plane of a semiconductor wafer to be processed. As shown in FIG. 1, such a plasma generating device 100 includes the planar antenna coil 110, a dielectric window 120, a gas distribution plate 130, the wafer to be processed 140, a vacuum chamber 150, an electrostatic chuck 160, and a lower electrode 170.
In operation, a radio frequency source (not shown) is used to provide a radio frequency current to the planar coil 110, typically via a radio frequency matching circuit (also not shown). The radio frequency current resonates through the planar coil 110, inducing a planar magnetic field within the vacuum chamber 150. At the same time, a process gas is introduced into the vacuum chamber 150 via the gas distribution plate 130, and the induced electric field ionizes the process gas to produce a plasma within the chamber 150. The plasma then impinges upon the wafer 140 (which is held in place by way of the electrostatic chuck 160) and processes (e.g., etches) the wafer 140 as desired. Another radio frequency, at a frequency which is different from that applied to the antenna coil, is typically applied to the lower electrode 170 to provide a negative DC bias voltage for ion bombardment.
FIGS. 2A and 2B depict two typical planar spiral coils 110a, 110b. As shown in FIG. 2A, a first planar coil 110a is constructed as a singular conductive element formed into a planar spiral and connected to radio frequency taps 205, 215 for connection to radio frequency circuitry. In FIG. 2B, an alternative planar coil 110b inter-connectors 225 and coupled at each end to radio frequency taps 205, 215.
As is well known in the art, the circular current pattern provided by such spiral coils creates toroidal-shaped plasmas which can in turn cause radial non-uniformity in etch rate at the wafer 140. In other words, the E-field inductively generated by the planar coil antenna 110 is generally azimuthal (having a radial component E.sub.r =0 and an azimuthal component E.sub..theta. .apprxeq.0), but zero at the center (E.sub.r =0 and E.sub..theta. =0). Thus, the coil antenna 110 produces a toroidal plasma with lower density in the center, and must rely on plasma diffusion (i.e., electrons and ions diffuse into the center) to provide reasonable uniformity at the center of the toroid. In certain applications, however, the uniformity provided by plasma diffusion is insufficient.
Further, such spiral coil antennas tend to make the gas distribution plate 130 susceptible to build up of polymer (a by-product of the etch process). This results from the fact that the relatively long lengths of coupling lines used to construct the planar antenna coils have significant electrical length at the radio frequency at which they typically operate. As a result, a standing wave exists on the coil so that the voltage and current vary periodically along the length of the coil. If the coil is grounded at the terminal end, the current at the terminal end is at a maximum value, and the voltage at the terminal end is zero. Proceeding along the coil toward the input, the voltage increases and the current decreases until, at 90 degrees of electrical length, the voltage is at a maximum and the current is at a minimum. However, such a degree of variation would result in a highly non-uniform plasma. Consequently, the planar coil is typically terminated with a capacitance such that the current in the coil is similar at both ends of the coil and increases to a maximum near the middle of the coil. Doing so provides a reasonably uniform plasma density as described above.
However, at the point in the coil where the current is at its maximum, the voltage is at its minimum and the voltage rises to nominally equal values (of opposite polarity) at each end of the coil. As a result, the voltage at the minimum point is quite low, and since the adjacent turn on either side of the minimum are at opposite polarities, some fraction of the electric field is confined between the adjacent turn and only the differential portion of the field penetrates the plasma. Since a certain minimum level of voltage is required to prevent excessive polymer deposition in many applications, the above described planar coils may be unsuitable in certain instances. For example, in addition to affecting etch selectivity of oxide to photoresist at the wafer 140, polymer build up can also cause particle problems if polymer flakes fall onto the wafer during processing.
Note that, although the terminating capacitor value can be varied, doing so only changes the position of the voltage null along the coil. Further, although the coil can be terminated with an inductance in order to provide the same polarity voltage along the coil length, a current null will exist somewhere in the middle of the coil (with the current traveling in opposite directions on either side of the null), and the resulting plasma density can be unacceptably low and non-uniform. Thus, there is a need for improved methods and apparatus for generating plasma in a radio frequency plasma coupling system.